As is known, the chloride ion in contact with metal produces a very unique form of corrosion called pitting. This form of attack affects most materials contemplated for use in certain environments such as sea water and certain chemical process industry media. While most forms of corrosion proceed at a predictable and uniform rate, pitting is characterized by its unpredictability. In most corrosive atmospheres, metal is uniformly dissolved with relatively uniform loss of gage from attack on all parts of the surface area of a sample. However, pitting is characterized in that it concentrates in specific and unpredictable parts of the metal surface, with attack concentrated in some few places by leaving the surrounding metal virtually untouched. Once initiated, the pitting process stimulates itself (i.e., the process is autocatalytic) concentrating the chloride ion into the initiated pit and accelerating the reaction rate.
In the past, austenitic stainless steels have been developed which are resistant to pitting by virtue of a relatively high level of chromium and especially a high level of molybdenum. One such alloy, for example, is described in Bieber et al. U.S. Pat. No. 3,547,625, issued Dec. 15, 1970. Other examples of austenitic stainless steels containing high levels of molybdenum and chromium are U.S. Pat. Nos. 3,726,668; 3,716,353 and 3,129,120. Unfortunately, producers have had difficulty in producing austenitic stainless steels with a high molybdenum content due to their poor hot-workability. For example, Type 334 stainless steel containing essentially no molybdenum is relatively easy to hot-work; Type 316 stainless steel containing 2% to 3% molybdenum has decreasing hot-workability characteristics; and Type 317 stainless steel containing 3% to 4% molybdenum is extremely difficult to hot-work with the result that certain steel concerns decline to produce it.
In the past, various alloying additions have been tried in an effort to improve hot-workability. Additions of up to 0.23% aluminum have been found to actually decrease hot-workability. Magnesium in the range of less than 0.001% to 0.06% tends to improve the hot-workability of austenitic stainless steels; however, magnesium is difficult to add to a melt with any degree of control of recovery and the workability is not materially improved.